Doors comprising a skeleton of wood stiles and rails covered on each side with a metal skin have been widely and successfully used for a substantial period of years. One example of a door of this construction which has had outstanding commercial success is provided by Pease U.S. Pat. No. 3,153,817 of 1964. The metal skins on the door of that patent include flanges along both sides thereof which overlap the edges of the wooden skeleton, and these flanges include additional flanges along their outer edges which are parallel with the skins and fit into a groove in the edge of the wood skeleton but are spaced from each other within that groove to provide an air gap minimizing heat transfer from one skin to the other.
A practical disadvantage of doors constructed in accordance with the Pease patent is that because both side edges of the door are covered with metal flanges, except for the air gap therebetween, the lateral dimensions of the door are fixed, and it is not possible to make any adjustment in those dimensions to accommodate irregularities in the door frame in which it is to be mounted. Also, it is standard manufacturing practice with such doors that the necessary openings in the flanges covering the lock side edge of the door to receive latch and lock hardware are cut at the factory, and the hardware to be installed in such a door is therefore practically limited to such hardware as will accurately fit those openings.
At least in part for each of these reasons, there is a substantial market for metal skin covered doors wherein the flanges on the sides of the skins do not fully cover the edges of the wood skeleton, and some wood projects beyond those flanges so that it can be planed or sanded as may be required to fit within a particular door frame, as well as slotted as needed to receive particular hardware. For example, Seely U.S. Pat. No. 4,152,876 of 1979 shows a metal faced door wherein the retaining flanges on the metal skins are set in slots in the sides of the stiles so that a portion of each of these stiles projects beyond the flanges and can be trimmed as needed to fit within a particular door frame, as well as slotted to receive a latch assembly.
There is a fire resistance limitation with doors constructed as shown in the Seely patent, as well as with similar doors wherein the flanges on the metal skins are retained in rabbets along the edges of the stiles, in that their fire resistance is undesirably low. For example, when subjected to a particular test procedure identified as ASTM E-152-81a, which rates in minutes the ability of each tested door to withstand those conditions, doors constructed in accordance with the Pease patent have a rating of 90 minutes.
In contrast, metal skin covered doors wherein substantial portions of the stiles are exposed have proved to be incapable of a rating higher than 20 minutes. This is an increasingly important consideration for the door making industry, because there is an increasing tendency for building codes to require specific fire resistant rating substantially higher than 20 minutes for door installations such, for example, as entry doors to hotel rooms, and between a residence and its attached garage, for which 45-minute ratings in a steel frame are often required.